++和--运算符重载的指针问题

#include<iostream>
using namespace std;
class Point
{
    public:
    Point(){}
    Point(int vx,int vy)
    {
        x=vx;
        y=vy;
    }
    void display()
    {
        cout<<"<"<<x<<","<<y<<">"<<endl;   
    }
    Point &  operator++();    //前置自增
    Point operator++(int);  //后置自增
    friend Point & operator--(Point &p);  //前置自减
    friend Point operator--(Point &p,int); //后置自减
    private:
        int x,y;

 };

 Point &Point::operator++()   //前置自增
{
     if(x<640)
     x++;
     if(y<480)
     y++;
     return *this;

 }

 Point Point::operator++(int) //后置自增
{
     Point temp(*this);
     if(x<640)
     x++;
     if(y<480)
     y++;
     return temp;

 }

 Point &operator--(Point &p)   //前置自减
{
     if(p.x>0)
     p.x--;
     if(p.y>0)
     p.y--;
     return p;

 }

 Point operator--(Point &p,int)    //后置自减
{
     Point temp(p);
     if(p.x>0)
     p.x--;
     if(p.y>0)
     p.y--;
     return temp;

 }

 int main()

 {
     Point p1(200,200),p2(300,300),p3(400,400),p4(50,50);
     
    cout<<"p1=";  //测试前置自增
    p1.display();
    ++p1;
    cout<<"++p1=";
    p1.display();
   
    cout<<"p2=";    //测试前置自减
    p2.display();
    ++p2;
    cout<<"++p2=";
    p2.display();
   
    cout<<"p3=";   //测试后置自增
    p3.display();
    p3++;
    cout<<"p3++=";
    p3.display();
   
    cout<<"p4=";    //测试后置自减
    p4.display();
    p4--;
    cout<<"p4--";
    p4.display();
   
    return 0;

 }